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Whereas in the past windows were generally single glazed consisting of just a single layer of glass, today there are many different options for glazing. A substantial amount of heat is lost through the window, and so double and triple glazed units have been developed to provide more insulation thereby improving the energy efficiency of a building.

Glass can also be tinted to reject sunlight, coated in a translucent film to increase energy efficiency, or be self-cleaning. It is most common for glazing to be clear glass; however, there are also many varied forms of etched, textured, frosted, stained or tinted glass for privacy or aesthetic purposes. For more information, see Glass manifestation.

An insulated glass unit (IGU) combines multiple glass panes into a single unit system, for example a window. Double glazed units are made of two sheets of glass with a cavity between them that is generally 4-20 mm which creates an insulating barrier. Triple glazed units have three sheets of glass. IGUs help combat heat loss, making a building warmer, quieter and cheaper to run.These are prefabricated panels fitted into a single planar system.

The sheets of glass are generally tempered (toughened) or laminated for safety. To keep the panes of glass apart, pane spacers are set around the inside edges. The more efficient windows use spacers that contain little-or-no metal, often known as 'warm edge' spacers.

The cavity between the panes can be a vacuum (the efficiency is dependent upon the quality of the sealing), or a heavy inert gas. This improves insulation and prevents condensation within the unit. Argon is the most commonly used gas, but Krypton or Xenon are more efficient and more expensive.

R-values are a measure of the thermal resistance of a material of a specific thickness, that is, its resistance to the transfer of heat across it. The higher the R-value of a material, the more effective it is as an insulator. Double-glazed units filled with air have an R-value of around R-2. Filling with Argon gas raises the value to R-3. Changing the glass to Low-E (see below), can raise the value to R-4. Triple glazed IGUs can have a value of R-5 or higher.

This is named after the modern process used to create large, thin, flat panels from molten glass which is floated onto a pool of molten metal such as tin. This process produces a very smooth sheet of glass with a highly consistent thickness.

This is a piece of float glass that has been cooled in a slow and controlled manner. The internal stresses within the sheet of glass are reduced by this process making the resulting glass stronger and less likely to break than it would otherwise be. There can be safety concerns using annealed glass as it can break into large jagged shards.

Tempering is the process by which annealed glass is heated in the same way as heat strengthened glass. The glass is cooled more rapidly which allows the internal portion of the glass to remain fluid for longer than the outer surfaces. This means that an equal amount of tensile and compressive stresses are formed across the glass which allows it to become in the order of four times as strong as annealed glass. This is used as a safetyglass, as it shatters into small granular pieces rather than sharp shards, reducing the risk of injury.

This is used as a means of testing glass panes that are to be used in safety critical situations, such as a glassrailing. Tempered glass panes are heated to a temperature of around 550 degrees Fahrenheit for a few hours. This causes any unstable nickel sulfide inclusions (imperfections that may cause spontaneous breakage of the pane) to expand disproportionately to the glass, making the glass break.

Laminated glass is used where glazing must remain intact if it is broken, either for safety or security reasons. It is made by fusing two or more layers of glass with inter-layers of polyvinyl butyral (PVB) through the use of heat and pressure. If it is made using heat strengthened glass, the pane will break into large pieces but will be held in the frame by the PVB inter-layer. If it is made from tempered glass, the sheet may fall out of the frame but will mostly stay together due to the inter-layer.

In cooler climates this means that long-wave infra-red radiation that builds up inside a building is reflected by the glass back into the space, rather than being absorbed by the glass and then partially re-radiated to the outside. This reduces heat loss and so the need for artificial heating.

The two main types of low-e coating are tin and silver. Tin oxide is applied to the glass at high temperatures to create a very hard and durable low-e coating. Silver coating must be enclosed within the glazingunit so that oxidation doesn't cause the degradation of the silver over time.

A transparent coating can be applied to glass during the manufacturing which reacts with the sun's UV rays to break down dirt and grime which forms on the outside of the windows, and when it rains, the decomposed dirt natural rinses away. The coating has hydrophilic properties which mean it attracts water over its entire surface, avoiding unsightly uneven water marks.

'Replacing the glazing whilst retaining an existing frame is not providing a controlled fitting, and so such work is not notifiable and does not have to meet the Part Lstandards, although where practical it would be sensible to do so.'